1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, though not exclusively, relates to a microminiature zoom lens.
2. Description of the Related Art
In recent years, optical equipment made up of an imaging module combined with a solid state imaging device such as a photographing lens, CCD, or CMOS is mounted on a portable terminal such as a cellular phone, and PDA. With such optical equipment, reduction in thickness of the entire equipment and in the imaging modules have been advanced to facilitate portability.
Conventional imaging modules, particularly imaging modules employed for cellular phones, mount a single-focus photographing lens, where reduction in thickness is readily realized with a relatively smaller number of lenses.
On the other hand, high-quality image technology has advanced, and cellular phones mounting an imaging device of 1 to 2 mega-pixels have appeared. Also, on the other hand, the desire has increased for zoom lenses as photographing lenses. As for zoom lenses corresponding to such small optical equipment, negative lens proactive type (negative lead) zoom lens systems where a negative lens is disposed on the object side have been discussed (Japanese Patent Laid-Open No. 2003-177314 (no corresponding overseas patent documents) and Japanese Patent Laid-Open No. 2004-4765 (no corresponding overseas patent documents)).
With Japanese Patent Laid-Open No. 2003-177314, a zoom lens made up of 3 lenses has been discussed where a 3-group configuration made up of negative, positive, and positive refractive power lens groups in order from the object side to the image side, the first and second lens groups are moved so as to reduce the interval between the first lens group and the second lens group, and increase the interval between the second lens group and the third lens group. Thus, reduction in size of the entire lens system is achieved by reducing the number of lenses.
Further, reduction in size of the entire lens system is realized while lengthening the exit pupil by configuring the third lens group with positive refractive power to be fixed at the time of zooming.
Also, Japanese Patent Laid-Open No. 2004-4765 discusses a zoom lens made up of three groups, and Japanese Patent Laid-Open No. 2003-177314 discusses a zoom lens made up of five or six lenses.
Upon light being cast into an imaging device with great deviance from vertical incidence, vignetting is caused due to the structure between the surface of the imaging device and the photo acceptance unit, so the zoom lenses discussed in Japanese Patent Laid-Open No. 2003-177314 and Japanese Patent Laid-Open No. 2004-4765 reduce vignetting from occurring.
Incidentally, in recent years, improvement of imaging devices has advanced, and an imaging device having a configuration where the surface thereof is disposed with an on-chip lens subjected to decentering, and an imaging device having a configuration wherein the above vignetting is hardly caused even if an incident angle is relatively great by providing an intra-layer lens within the imaging device, have been discussed. Thus, for a zoom type lens where the exit pupil is relatively short, a 3-group zoom lens made up of negative, positive, and negative refractive power lens groups in order from the object side to the image side has been discussed (U.S. Pat. Nos. 5,283,693, USP No. 5,357,374, and USP No. 5,268,792).
U.S. Pat. No. 5,283,693 discusses a zoom lens made up of 5 lenses in all, where the focal distance of the entire system is changed by changing the interval between the respective lens groups.
U.S. Pat. No. 5,357,374 discusses a zoom lens, which changes the focal distance by changing the interval between the respective lens groups, where a first lens group is made up of a single negative lens, a second lens group is made up of a positive lens of which the surface on an object side is a concave meniscus shape, and a third lens group is made up of a negative lens of which the surface on an object side is a concave meniscus shape.
U.S. Pat. No. 5,268,792 discusses a zoom lens made up of three through six lenses in all.
It is difficult for the respective embodiments of the zoom lens discussed in Japanese Patent Laid-Open No. 2003-177314 to handle a sensor of a mega-pixel or greater from the aspect of optical performance.
The lens performance in the respective embodiments in Japanese Patent Laid-Open No. 2004-4765 can handle mega-pixel sensors, but is designed under the condition of a collapsible camera, so that the total length is long as an optical system, and sufficient reduction in size cannot be always realized.
With the zoom lenses in Japanese Patent Laid-Open No. 2003-177314 and Japanese Patent Laid-Open No. 2004-4765, optical performance can be improved by further increasing the number of lenses to be configured.
However, upon the exit pupil being lengthened, the effective diameter of the lens system becomes closer to the sensor size overall, and consequently, the thickness of the lens system needs to be increased, resulting in the total lens length being long.
With U.S. Pat. No. 5,283,693, the number of lenses to be configured is small, and is a 3-group zoom lens made up of negative, positive, and negative refractive power lens groups, but this zoom lens is weak in the refractive power of the third lens group, so that contribution to reduction in size of the lens system due to the negative lens of the third lens group is not always sufficient.
Also, even with the zoom lens in U.S. Pat. No. 5,357,374, the refractive power of the third lens group is very weak, so the same problem as mentioned above occurs. Also, the respective lens groups have a single lens configuration, so sensitivity is very high.
The zoom lens in U.S. Pat. No. 5,268,792 is short in back focus for use as a viewfinder camera, so that it is difficult to obtain minimum required back focus in the event of applying this to an imaging device.
In general, when the number of lenses in the respective lens groups making up a zoom lens is great, the length on the optic axis of each lens group becomes long, and also when the amount of movement in zooming and focusing of the respective lens groups is great, the total lens length becomes long, so it becomes difficult to realize reduction in size of the entire lens system.